All right so maybe we were a little outnumbered by the World of Warcraft players, and maybe a little part of me died every time someone screamed "For the Horde!" in my ear, but there was still a longer line to play StarCraft II at BlizzCon 2009 than there was to play World of Warcraft: Cataclysm. Take that, Arthas! But in all seriousness, we got some hands-on time with StarCraft II (as much as we could in a weekend) and while the changes are small, the game is coming along well.
Specifically the single player has really come into its own. StarCraft II: Wings of Liberty stays true to its roots with original RTS-style gameplay, forsaking the standard "defeat this enemy" mission style and adding more missions where objectives and strategic timing play a larger role. Another great feature is the addition of environmental effects like lava overflows which actually affect the player's gameplay directly by killing unwary units. Two missions were available this year, including Evacuation of Agria and Monolyth. Both were, of course, from the perspective of Raynor's Raiders and had time-based objectives to complete.
Something that has definitely added a new level to single player in StarCraft II is the addition of achievements. Sure, other games have rewards for 100 kills but SC2 has achievements for each mission that determine your reward at the end and really add strategic value and makes it more challenging. It also extends the game's playability by quite a bit. Finishing a mission with all your achievements intact is a great test.
As far as multiplayer gameplay is concerned, there's not much to critique. The Zerg are the epitome of nastiness and the Ultralisk is just as much fun, if not more, than it was in the original. They have a slightly steeper learning curve than the rest of the races but it's worth the effort. The new macro-management features increase the potential for the player to achieve higher levels of play with minimum effort. StarCraft II is a game that will start easy and gradually become harder as your skill level improves.
Of course there's going to be minor balance issues for the next couple of years but really the game could be shipped whenever Blizzard gives the go-ahead. Receiving a copy of the game in its current build would be enough to satisfy 99 percent of players and have them begging for the expansion. The game is sleek, has the best UI of any RTS I've played (C&C needs a revamp), and its new gameplay mechanics manage to avoid the gimmicks and provide the player with a unique experience.